Berkeley Springs has historical significance, being a spot of local mineral waters that the country's founding fathers bathed in. Today, you'll find spas all over town offering treatments with these waters, but the Roman Baths at the state park are some of the cheapest.
That said, these baths are quite basic and no-frills. Essentially, you'll get a private room with a big bathtub (easily big enough for 4) filled with hot, mineral water. There was no bench in the pool, so you couldn't really sit while soaking (the stair entry wasn't quite conducive to sitting for more than a minute or two).
There isn't a lot of floor space outside the bath, so you'll either need to change in the bathroom area (therefore walking through the hallway/public area in your bathing suit & towel) or try to change in your private bath room without dropping anything in the floor. We found this hard to manage with two people.